1. Field of the Invention
The present invention relates to the adjustment of a filter device and, in particular, to the adjustment of a tunable chromatic dispersion compensating device, used in an optical transmission system.
2. Description of the Related Art
In recent years, work has been proceeding to increase the capacity of optical wavelength division multiplexing (WDM) transmission systems. There are two main ways of increasing the capacity: one is to increase the number of wavelengths, and the other is to increase the bit rate. As for the bit rate, 10-Gb/s optical transmission systems have already been commercially implemented, and the development of 40-Gb/s optical transmission systems is currently under way.
As the bit rate increases, the influence of the degradation of optical signal waveforms due to chromatic dispersion of optical fibers increase, and degradation in transmission quality becomes more pronounced. That is, the dispersion tolerance decreases, decreasing the allowable range of the amount of dispersion. This necessitates the provision of an automatic dispersion compensation system that can cope with the variation of chromatic dispersion with temperature, aging, and the like, in addition to the dispersion-compensating fibers used for dispersion compensation in traditional systems.
An example of a tunable chromatic dispersion compensating device used in such an automatic dispersion compensation system is the VIPA (Virtually Imaged Phased Array) described in a document presented by M. Shirasaki et al., “Dispersion Compensation Using the Virtually Imaged Phased Array,” APCC/OECC '99, pp. 1367–1370, 1999.
In the automatic dispersion compensation method using the VIPA, when the amount of dispersion compensation is changed by moving a mirror, the center wavelength of the passband also changes at the same time. As a result, the optical signal spectrum changes, affecting the transmission quality.
Further, as the optical transmission system includes other filter devices, such as optical MUX/DEMUX (multiplexing/demultiplexing) filters, the total passband is determined by the combination of these filter devices and the VIPA. When the total passband determined by this combination is considered, there are cases where optimum transmission quality cannot be obtained even when the passband center of the VIPA is aligned with the signal wavelength center.
When a filter device having the above-described passband characteristic is used in the optical transmission system, if the amount of dispersion compensation alone is simply tuned so as to provide the best result, the transmission quality may degrade in certain cases.
Furthermore, if the center wavelength of this filter device is correctly controlled, it does not necessarily follow that the passband center wavelengths of other filter devices placed in the transmission path, such as the optical MUX/DEMUX filters, are also set correctly. The same can be said of the wavelength of the light source. If any of these center wavelengths deviates from the correct value, transmission quality degradation will result. Further, among these devices, there may be a device or devices whose center wavelengths cannot be controlled; therefore, when the total characteristic of the transmission path is considered, adjusting to the specified wavelength may not always lead to the best result.